IOS視頻添加背景音樂同時保留原音

//抽取原視頻的音頻與須要的音樂混合 -(void)addmusic:(id)sender { [MBProgressHUDshowHUDAddedTo:self.viewanimated:YES]; AVMutableComposition *composition =[AVMutableCompositioncomposition]; audioMixParams =[

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